My bet would be 1.3 million.
It is a prefecture-level city. In China, that means tier-II city. These cities are not near 5 million in size although some are growing close to it ... so I can't be sure. Although Wikipedia works pretty well and is generally as consistent as Brittanica, in this case, I consulted Britannica. Unfortunately, it says that city had 450k people in 2002. There is no way a 10-fold increase happened in 20 years and the world did not hear more about it although theoretically possible. It is also an ancient city (well, China is ancient). See here:
Britannica entry
It would be great to hear from first-hand visitors. Unfortunately, the closest I have been to is Shanghai.
On a different tangent, Britannica and print post and journalism will cease. Where do we check then?